Generally, color laser printers have operated with a fixed resolution, often specified in terms of dots per inch (dpi). For example, 300 dpi and 600 dpi laser printers are commonly found in office environments. In such laser printers the spot size (i.e., the size of the printed dot) at a print medium remains fixed.
A superior quality photoprint requires high resolution (about 600 or more dpi). A good quality photoprint can be made with the lower resolution (300 dpi) printers.
The higher the resolution, the more pixels are associated with a given size image area and the more data information needs to be processed for the given size image area. While printing of monochromatic text requires only two intensity levels (zero and full saturation) and only one color (black), color imaging requires the application of multiple colors (red, green blue, for example) in many levels of intensity (256 or 512, or even 16,000,000 levels, for example). Thus, a laser printer capable of printing photographic quality color pictures must process a very large amount of image data describing an image.
It is desirable for a printer to produce a large volume of prints per a given unit of time. If a high speed printing (about 5 or more prints per second) requirement is added to the requirement of high resolution, multicolor printing , it becomes difficult to electronically process this amount of data in a short period of time. A lower resolution printer (about 300 dpi) would satisfy the faster printing requirement by decreasing the amount of data (per second) needed to be processed, but would also decrease the image quality of all prints.
The industry recognized that many print jobs do not require high resolution and, that overall productivity would be increased by a printer capable of both fast production of low resolution prints, and slower production of high resolution prints. However, at a lower resolution, having fewer exposed pixels per inch leaves white spaces between the exposed pixels. (FIGS. 1A and 1B illustrate schematically a section of a high and a low resolution photoprint, respectively.) These white spaces degrade the quality of a photoprint, giving it a "washed out" appearance.
It has been proposed that LED printers utilize two adjacent LEDs to expose each of two adjacent (cross track) pixels with the same information (processing data for only half of the pixels and cutting the amount of time required for data processing in half). This technique would eliminate white spaces between the exposed pixels, but only in a cross track (i.e., line) direction. It would leave white lines of unexposed pixels (white space) in the in-track direction between scan lines A, B (FIG. 1C).
A laser printer, to facilitate faster prints, may expose two adjacent cross track pixels with the identical data so as to reduce the amount of unexposed pixels and thus minimize the amount of "white spaces". This approach, however minimizes the "white space" in a cross track (i.e., line) direction only. It would still leave a line of unexposed pixels between the two lines of exposed pixels. This problem could be remedied by exposing an additional line of pixels in the in-track direction between the lines A and B and making this exposure identical to that of the line A--i.e., by printing the same line twice. This is illustrated in FIG. 1D. This technique, by printing the same line twice, eliminates extra image processing for every other line. However, because this technique still results in printing twice as many lines, it would take twice as long to make each photoprint.